Revert "Remove the default None for data_path, which is a required param."
Revert "Remove the default None for data_path, which is a required param."

This reverts commit 6d829caa15d4c37e1adc42368f38833c6ae2c264.

tensorflow/examples,tensorflow/examples,tensorflow/examples,tensorflow/examples,tensorflow/examples,tensorflow/examples,tensorflow/examples,tensorflow/examples,tensorflow/examples